1. Field of the Invention
The present invention relates to a method and apparatus for adjusting a density detecting device which is used for an image forming apparatus for forming an image by an electrophotographic process, for example, an electrostatic copying machine, and which device is for outputting density data utilized in adjusting the image forming conditions such as the amount of charge, the amount of exposure and the developing bias so as to keep the formed image high in quality.
2. Description of the Related Art
In the electrostatic copying machine, a copy image is formed in the following manner. Specifically, a real original which is put on a transparent platen to reproduce the image thereof is illuminated and scanned. Reflected light from the real original is introduced into a photosensitive drum which is rotated in synchronization with the illumination and scanning. As a result, the photosensitive drum is exposed. The surface of the photosensitive drum before the exposure is uniformly charged by a charger. An electrostatic latent image corresponding to the real original is formed on the surface of the photosensitive drum by selective charge elimination caused by the exposure.
The formed electrostatic latent image is developed into a toner image by a developing device to which toner is supplied from a toner hopper. The toner image is transferred onto copy paper by corona discharges in a transferring corona discharger. The copy paper on which the toner image has been transferred is introduced into a fixing device, where the toner is fixed to the copy paper, thereby completing copying.
An attempt to stably obtain an image high in quality in the above described electrostatic copying machine brings about the necessity of suitably adjusting the image forming conditions such as the amount of exposure and the amount of charge of the photosensitive drum, the developing bias and the amount of toner to be supplied to the developing device.
The image forming conditions are adjusted for each predetermined period, for example, at the time of maintenance. In adjusting the image forming conditions, a pure white or solid black pseudo original (a reference density original) which is arranged in a region other than a region where the real original is illuminated and scanned is experimentally illuminated, and a toner image corresponding to the pseudo original is formed. At this time, the amount of exposure, the surface potential, the density of the toner image on the surface of the photosensitive drum, and the like are detected, and the image forming conditions are automatically adjusted on the basis of the results of the detection. Specifically, where the pure white pseudo original is illuminated to form a toner image, if so-called fog is detected on the basis of the detected toner image density, the amount of exposure is increased. On the other hand, where the solid black pseudo original is illuminated to form a toner image, if it is judged that the density is insufficient on the basis of the results of the detection of the toner image density, toner is automatically supplied to the developing device from the toner hopper.
A reflection type photosensor which is constituted by a pair of a light emitting element and a light receiving element arranged opposed to the photosensitive drum is generally applied to the detection of the density of the toner image on the surface of the photosensitive drum. Specifically, light of a previously set amount is irradiated onto the photosensitive drum from the light emitting element, and the amount of light received by the light receiving element which corresponds to the amount of light reflected from the photosensitive drum is detected. Since the amount of the reflected light corresponds to the density of the toner image on the surface of the photosensitive drum, it is possible to detect the density of the toner image on the surface of the photosensitive drum if the amount of received light is detected.
The amount of light irradiated onto the photo sensitive drum is set to either an amount for low density or an amount for high density, for example. The amount for low density is relatively small, while the amount for high density is relatively large. The amount for low density is applied for fog detection and the amount for high density is applied for solid black detection.
A toner image density used for adjusting the image forming conditions is detected by illuminating the pseudo original, as described above. On the other hand, the amount of reflected light differs due to mechanical factors or the like of the electrophotographic copying machine between a case where the pseudo original is illuminated and a case where the real original is illuminated and scanned. The factors of the difference in the amount of reflected light include the difference in the set position, the difference in color, and the difference in the positional relationship with a light modulating plate between the pseudo original and the real original.
For example, in the electrostatic copying machine, if the pseudo original is arranged in a position closer to the photosensitive drum, as compared with the real original, the amount of reflected light in a case where the pseudo original is illuminated becomes smaller than that in a case where the real original is illuminated. The reason for this is that a light source for illuminating and scanning the real original is generally designed so that light to be irradiated is converged on the surface of the real original.
Consequently, a toner image density corresponding to a pure white region of the real original is lower than a toner image density corresponding to the pseudo original on which a pure white image is formed. Hence, even under the image forming conditions (for example, the amount of exposure) properly adjusted so that fog is removed in the toner image density corresponding to the real original, a toner image of relatively high density may be formed when the pseudo original is illuminated. Thus, in fog detection by detecting the density of the toner image corresponding to the pseudo original, the density of the toner image may not always be detected correctly even with light of the amount for low density irradiated onto the photoreceptor from the reflection type photo sensor. In some machines, the amount for high density may be preferable for fog detection utilizing the pseudo original. The image forming conditions cannot be effectively adjusted unless the density of the toner image corresponding to the pseudo original is accurately detected.